In general, in radio frequency transmission systems (RF systems) communication equipment such as RF communication receivers (front-end circuitry) usually comprise in its signal path a RF front-end filter to suppress the power contained in unwanted transmission channels which are adjacent to a particular transmission channel which is the wanted channel. The suppression of power of the unwanted adjacent channels supports to meet linearity requirements of the RF communication receiver including the RF front-end filter, and benefits precise channel selectivity and reduces power consumption of the entire communication equipment in which the RF communication receiver is installed.
The RF communication equipment, and especially the terrestrial ones, have to cope with a large input frequency range (RF frequency range) which typically extents from 50 MHz to 1 GHz. This means that the filter must be a tracked band-pass filter (BPF) which needs to be accurately tuned at the wanted (desired) frequency, that is, at frequencies of the wanted channel. Most of the tuner devices use a super-heterodyne like architecture, according to which a RF signal (radio frequency signal) is down-converted into an intermediate frequency (IF). A local oscillator (LO) of the transmission system is generally a voltage-controlled oscillator (VCO) controlled by a PLL circuit (Phase Locked Loop circuit) and usually tuned by an external varactor. To track the radio frequency the RF filter must have a varactor which is of the same time as the VCO one, and the voltage applied on the VCO varactor must be applied on the RF filter varactor. In this case, the same technology for both the filter and the voltage controlled oscillator VCO is provided and leads to an accurate tracking over the whole frequency range. The parameter of interest, i.e. the power of the wanted channel can exactly be calibrated.
Patent document US 2003/0176174 discloses a method of operating a RF receiver (Radio Frequency receiver) of a communication equipment, wherein for calibration purposes a calibration signal is generated and is injected into a low noise amplifier of the RF receiver. The down-converted response of the receiver at a plurality of different frequencies of the calibration signal is measured. The measured response of the RF receiver represents the output power corresponding to each test signal of the calibration signal in the baseband, and after RF mixing is performed, it is determined, which of the results has produced the highest output power in the baseband. This process is, however, not sufficiently accurate and does not allow to take a decision in real time.
Moreover, signals are transmitted by the communication equipment according to a baseband with different frequencies. In receiving systems or communication equipment the signal paths (or at least a part of it) when transmitting signals of different frequencies often introduce a tilt in the wanted channel. In particular, the transmission of plural signals each having different frequencies may vary when the signal pass through the signal path, and the tilt is the variation of the signal amplitude in function of the frequency of the respective transmitted signal. That is, when two signals having different frequencies are handled and transmitted, and signal levels of such signals are measured, the difference between the measurements of the two signals having different frequencies represents the tilt of these signals. The tilt of the channel represents the difference between measurements of signals including a signal having the lowest possible frequency and a signal having the highest possible frequency.
In this connection, FIG. 4 shows a front-end circuitry 1 which is in general used to for handling and collecting input data and signals. The front-end circuitry 1 includes a filter unit which is provided in the form of a band-pass filter B. The band-pass filter B is adapted for filtering a frequency band depending upon a value of an applied control voltage for frequency selection (connection C). The front-end circuitry 1 may be arranged or implanted in a RF communication equipment, such as a mobile telephone.
Radio frequency signals to be handled by the front-end circuitry 1 are input to the input terminal IN and are treated by an input amplifier IA. The output signal of the input amplifier IA is transmitted to the filter B, and between the input amplifier IA and the filter B a connection point CP is provided at which a frequency signal fi can be injected. That is, the frequency signal fi is introduce into the signal path of the front-end circuitry 1 and input to the band-pass filter B.
A filter calibration unit FC is connected to the output terminal of the band-pass filter B for sensing the output signal thereof. The filter calibration unit FC provides the frequency signal fi to be input to the band-pass filter B, and also provides a further signal in the form of a control voltage which is input to the band-pass filter B for setting the band-pass filter B to a certain selected frequency.
The output signal of the band-pass filter B is mixed in a mixing unit M with a frequency signal FLO (from a local oscillator), and the resulting signal is fed to the rest FE of the front-end circuitry. This rest FE of the front-end circuitry provides an output for further data evaluation.
In the above-described front-end circuitry 1, when plural RF signals are handled with different frequencies within a particular channel, usually a tilt in such a wanted channel is introduced and leads to distortion of the signal evaluation and a degraded performance of the front-end circuitry 1.
In order to ensure a signal transmission (specifically a propagation through the signal path) with at least low distortion, and specifically for good channel reception, the tilt of the signals or of the transmission channel (wanted channel) should be minimized without affecting the group delay.